Kauaʻi's climate varies dramatically by coast. The south shore around Poʻipū averages around 300 days of sunshine per year and is warm and dry year-round. The north shore is lush because it is wet — expect regular rain even in summer, with heavy downpours from October through April. The wet interior around Mount Waialeale receives rain virtually every day of the year.
Summer (June–September) is the best time for the north shore: calmer seas allow kayaking the Nā Pali Coast, the Kalalau Trail is at its best, and Hanalei Bay is safe for swimming. This is also the peak tourist season, when accommodation is most expensive and the Kalalau Trail permits sell out fast — book months ahead.
Winter (November–March) brings large swells to the north and west shores, making surfing excellent and swimming dangerous. The south coast is more reliable for sun in this period. Whale-watching season runs from December to April, with humpback whales visible from the coastline and boat tours.
The spring and autumn shoulder seasons offer a reasonable balance — fewer visitors, lower prices, and weather that is generally good on the south shore.